The purpose of this study is to evaluate the effects of cutaneous stimulation on motoneuron excitability in man. An experimental protocol has been developed for activating cutaneous afferents in lieu of muscle afferents. The experiments use both electrical and mechanical stimuli, which when delivered alone give restricted information on motoneuron excitability changes. When used together, complementary results are obtained from which inferences can be made about the selective effects of cutaneous afferent input. The magnitude of cutaneous effects is quantified by using a peristimulus time histogram technique for evaluating changes in motor unit firing. The effects of cutaneous input on the serial dependence of interspike intervals of soleus motor unit spike trains are evaluated by a statistical technique for constructing the serial correlogram. The results of this study have both physiological and clinical implications. Physiologically, the results provide a quantitative assessment of the potency of cutaneous afferent input onto motoneurons in man during low level isometric contractions. These findings therefore reveal the influence of a specific group of afferents on motoneuron excitability under specific contractile conditions. The results may also provide a baseline level of reference for evaluating more complex motor control problems such as the regulation of spinal reflex gating by skin afferent activation. Clinically, activation of cutaneous afferents represents one of the most prevalent forms of stimulation used in the treatment of patients with abnormal muscle tone. Presently, the type of stimuli used and methods of application are based almost exclusively on clinical observations. The results from this study will extend upon these observations and provide experimental verification of the effect of cutaneous stimulation on motoneuron excitability. Such findings may eventually aid in the modification of existing therapeutic techniques and/or development of new procedures. The work done in this grant will provide the foundation for a long term commitment to the study of cutaneous afferent effects on motoneuron excitability. Future studies may focus on the specific receptors involved, the role of skin afferents in spinal reflex gating, verification of these findings in motor units of different muscles, and the evaluation of a neurologically impaired population such as the stoke patient.